A block of the periodic table of elements is a set of adjacent groups. The term appears to have been first used by Charles Janet.[1] The respective highest-energy electrons in each element in a block belong to the same atomic orbital type. Each block is named after its characteristic orbital; thus, the blocks are:

The block names (s, p, d, f and g) are derived from the quality of the spectroscopic lines of the associated atomic orbitals: sharp, principal, diffuse and fundamental, the rest being named in alphabetical order from g onwards, omitting j.[2][3] Blocks are sometimes called families.

The following is the order for filling the "subshell" orbitals, according to the Aufbau principle, which also gives the linear order of the "blocks" (as atomic number increases) in the periodic table:

The "periodic" nature of the filling of orbitals, as well as emergence of the s, p, d and f "blocks" is more obvious, if this order of filling is given in matrix form, with increasing principal quantum numbers starting the new rows ("periods") in the matrix. Then, each subshell (composed of the first two quantum numbers) is repeated as many times as required for each pair of electrons it may contain. The result is a compressed periodic table, with each entry representing two successive elements:

There is an approximate correspondence between this nomenclature of blocks, based on electronic configuration, and groupings of elements based on chemical properties. The s-block and p-block together are usually considered as the main group elements, the d-block corresponds to the transition metals, and the f-block are the lanthanides and the actinides. However, not everyone agrees on the exact membership of each set of elements, so that for example the Group 12 elements Zn, Cd and Hg are considered as main group by some scientists and transition metals by others. Groups (columns) in the f-block (between groups 2 and 3) are not numbered.

In periodic tables organized by blocks, helium is placed next to hydrogen, instead of on top of neon as in tables organized by chemical properties. This is because helium is in the s-block, with its outer (and only) electrons in the 1s atomic orbital. In addition to the blocks listed in this table, there is a hypothetical g-block which is not pictured here. The g-block elements can be seen in the expanded extended periodic table. Also, lutetium and lawrencium are placed under scandium and yttrium to reflect their status as d-block elements[4] (although it has been argued that lanthanum and actinium should instead hold these positions, as they have no electrons in the 4f and 5f orbitals, respectively, while lutetium and lawrencium do).[5]